1. Field of the Invention
The present invention relates to a small size broadband ultra high frequency antenna used as a car telephone antenna, etc., and more particularly to improvements in the voltage standing wave ratio (hereinafter referred to as VSWR) of such antennas.
2. Prior Art
In general, the major characteristics which affect the performance of small size antennas are gain (directional characteristics) and VSWR. Of the foregoing factors, gain is almost invariably determined by the dimension of the antenna element. However, the VSWR characteristics vary largely depending on the engineering design of the internal structure.
Conventional small size antennas for ultra high frequency (UHF) have generally be relatively unsatisfactory in VSWR characteristics. Particularly, small size ultra high frequency antennas have serious disadvantages since their frequency bands are narrow.
There are several methods of expanding the frequency band. One of the methods is to increase the diameter of the antenna element and lower the "Q" of the antenna characteristics. This method can broaden the VSWR characteristics. Another method is to broaden the frequency band width by inserting two or more stages of matching transformers composed of distributed constant type 1/4 wave length impedance transducers, between the antenna and feeder line. With this method, the characteristic impedances of the respective stages can be set to become Wagner type characteristics or Chebyshev type characteristics.
FIG. 1 shows the VSWR characteristics of a skirt form dipole antenna as an example. In the FIGURE, a diagram for facillitating understanding of the structure of the antenna is also shown. As seen in FIG. 1, when an antenna element 1 is widened in its diameter as indicated by the broken line, by employing the above mentioned first method, the VSWR characteristics are changed from those represented by the solid line to that shown by the broken line. As a result, the VSWR characteristics broaden, and the frequency band is expanded.
Further, although it is not shown in the FIGURE, when using the second method, two stages of impedance transducers (matching transformers) are interposed between the antenna element 1 and a coaxial feeder line 2 for setting the respective characteristic impedances to becomes: EQU Z.sub.m.sbsb.1 =Z.sub.o 3/4.multidot.Z.sub.a 1/4, Z.sub.m.sbsb.2 =Z.sub.o 1/4.multidot.Z.sub.a 3/4
the VSWR becomes "1" in center frequency, thus obtaining Wagner's characteristics. In this manner, the closer the apparent antenna impedance is brought to the characteristic impedance ZO of the feeder line, the more the VSWR is improved by getting closer to "1".
However, the antennas in conventional use which have had the foregoing improved methods applied to them have the following problems. Particularly, in the antennas using the first method, the diameter of the antenna element is increased. Accordingly, such an antenna element cannot be used as a portable antenna, an enclosed antenna, etc., since these types of antennas need to be small in diameter in view of their function. Consequently, the first method makes it not feasible to construct, for example, a broad-band portable antenna and a broad-band enclosed antenna. Because of the plural number of stages of matching transformers used in the second method, antennas which use this method not only have a complicated structure but also the total length of the antenna is increased. Thus, the second method makes it impossible to construct a broad-band antenna which is short in length.